Widespread standardization of analog cellular communications frequencies and parameters, such as provided in North America by Telecommunications Industry Association/Electronics Industry Association Interim Standard 91 (TIA/EIA IS-91), Mobile Station-Base Station Compatibility Standard for 800 MHZ Analog Cellular, for the Advanced Mobile Telephone System (AMPS), permits the gradual introduction of digital transmission equipment into selected areas without wholesale replacement of the first-generation analog system. The ultimate goal is to substitute the superior signal quality offered by the second- and later generation digital systems for the entire analog system. However, this changeover is expected to occur over a period of several years and, in the meantime, digital transmissions will not be available in all areas, requiring continued use of the analog system. Channels in a given geographical area may be assigned to either digital or analog, while in other areas, the two signals may share channels. The North American Time Division Multiple Access (TDMA) standards, TIA/EIA IS-54 (Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard)IS-136 (TDMA Cellular/PCS Radio Interface--Mobile Station--Base Station Compatibility--Digital Control Channel), enhance, rather than replace, the analog cellular technology, making dual-mode operation possible. This is also the case according to the Global System for Mobile communication standard, TIA/EIA IS-129 (Interworking/Interoperability Between DCS 1900 and IS-41 Based Maps for 1800 MHZ Personal Communications Systems). For Code Division Multiple Access (CDMA), as defined in TIA/EIA IS-95 (Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System), although the broad spectrum can be shared, certain analog frequency channels must be devoted to the CDMA system, which provides an overall improvement in spectral efficiency.
In order to take advantage of digital systems, when available, while still assuring network coverage in areas on the fringes or outside of the digital coverage areas, dual-mode mobile stations and wireless networks are becoming increasingly common. In a dual-mode environment, a mobile station (MS) may at any given time obtain service from either a digital or an analog system, depending on a number of different conditions, including geographical location relative to a base station (BS), roaming status, and scan period.
When a mobile station is initialized, a system acquisition process occurs. The IS-95 standard requires that the MS choose the frequency band to use, A or B band, and the modulation to use, analog or CDMA system. The frequency band options can be chosen independently of the modulation options. Generally, the preferences are programmed into the memory of the MS when service is activated. Some typical preferences, as defined by the IS-95 standard, are (1) System A only; (2) System B only; (3) System A preferred (if No Service can choose System B); (4) System B preferred (if No Service can choose System A); (5) CDMA only; (6) Analog only; (7) CDMA preferred (if No Service can choose Analog operation); and (8) Analog preferred (if No Service can choose CDMA). Certain MS manufacturers provide programmable system selection capability in the MSs, however, this system selection is volatile and, after a power cycle, the selection will revert to its default selection.
During system acquisition the MS attempts to receive the signal transmitted by a base station providing service coverage to the geographic area where the MS is located. Without specified preferences, the MS will select whichever of the analog or digital signals is stronger. The MS may be able to receive either the analog or digital signal equally well, however, due to the increased capacity of digital systems, where both and analog and digital service are available, network service operators typically prefer that the MS will acquire and use the digital system. The system determination process can be programmed with this preference to maximize the number of users on the preferred network.
When digital service is available, the dual-mode MS would normally start a call on a digital channel. The analog channels are then reserved for analog-only MSs. A disadvantage with this preference is the potentially longer acquisition time required for digital systems, particularly with CDMA systems. Until acquisition occurs, the user of the MS cannot place his or her call. Thus, the interest of the user in having quick access to the system competes with the interest the carrier has in shifting the network loading from the analog system to the digital system.
Another problem occurs when the MS is being operated in fringe areas of coverage of one of the systems. As the MS moves out of the coverage area of the currently-selected system it must acquire the other system (usually the analog system), whether the user is attempting to place a call and is already involved in a call. When the MS moves back into the coverage area it may reacquire the original system if it is programmed as the preferred system. During attempts to acquire or reacquire the preferred system, new calls cannot be made or received, consequently, in a boundary area, the phone may be unusable at any given time.
In addition to the MS being programmed with preferences on service acquisition, the MS is also programmed with predetermined time intervals for attempting to obtain service, i.e., a "re-scan timer". After attempting to acquire service from one system for the specified time period the MS will attempt to acquire service from the other system. There is no standard to allow individual network service carriers to specify a re-scan timer for setting the duration of the attempted digital channel acquisition before going back to the analog system. Because dual-mode digital deployment occurs in many different markets and terrains, it is unlikely that one re-scan timer would be suitable for all service providers. It is also undesirable for MSs to repeatedly leave the analog idle state to attempt to re-acquire if there is no digital coverage available since, during re-scan, the MS is unable to receive calls on the analog system. The desired frequency of re-scan will, thus, be different for different providers and MS users based upon different criteria. For example, some network providers will be more concerned with call probability and/or quality, while others may have capacity constraints within the analog system which encourage them to move MSs off of the analog channel.
There are many factors which will effect the optimum selection of parameters for system determination. The balancing of factors may vary from carrier to carrier, and may be different for categories of users based on their amount of usage, areas traveled, and type of service plan purchased. With so many variables, the application of a single system determination algorithm cannot be used to optimize system utilization and service quality for all users. Further, since the variables and their respective levels of importance are likely to change with time, limiting the setting of the system preferences to the MS activation procedure will result in MSs becoming outdated when the changes are made. Accordingly, a need remains for determining preferences for system determination which can be personalized for individual network subscribers and can be adapted to fit the network's requirements, which may change with time, and which can still be readily implemented.